This type of rotary damper generally includes a damper body including a receiving hole with a bottom portion formed therein, a rotor rotatably fitted in an opening side end portion of the receiving hole, a piston movably disposed in a portion of the receiving hole between the rotor and the bottom potion, and movement means that causes the piston to be moved according to the rotation of the rotor. Inner space of the receiving hole between the rotor and the bottom portion is divided by the piston into a first chamber and a second chamber. The first and second chambers are filled with fluid such as viscose fluid. The movement means includes a cam mechanism disposed between the rotor and the piston and a coil spring that biases the piston toward the rotor. The cam mechanism allows the piston to be moved by the coil spring toward the rotor when the rotor is rotated in one direction. On the other hand, when the rotor is rotated in the other direction, the cam mechanism causes the piston to be moved in a direction away from the rotor against a biasing force of the coil spring.
When the piston is moved toward the rotor, the viscose fluid in the first chamber flows into the second chamber. Flow resistance of the viscose fluid at this time limits the speed of rotation of the rotor in the one direction to a low speed. When the piston is moved to the other direction, the viscose fluid in the second chamber flows into the first chamber. The flow resistance at this time is kept to be negligibly small. Therefore, the rotor can be rotated in the other direction at a high speed.
When the rotary damper described above is used in a toilet, for example, the damper body is fixed to either one of a toilet body and a toilet lid and the rotor is fixed to the other. In this case, the damper body and the rotor are fixed to the toilet body and the toilet lid such that the rotation speed of the toilet lid is limited to a low speed when the toilet lid is rotated in a closing direction and the toilet lid can be rotated at a high speed when rotated in an opening direction.
When the toilet lid is rotated through about 90 degrees from a closed position and the piston is moved to a predetermined position toward the rotor, the piston becomes rotatable in the one direction. As a result, the rotor becomes rotatable together with the piston in the one direction, allowing the toilet lid to be rotated through more than 90 degrees. Moreover, the coil spring rotationally biases the piston in the one direction. Therefore, after being rotated through 90 degrees from the closed position, the toilet lid is further rotated in the opening direction by the rotational biasing force of the coil spring. The toilet lid is stopped when it is abutted against a tank disposed in the toilet (refer to Patent Document 1).